Understanding changes in past climate requires effective temperature proxies. However, few existing palaeothermometers focus on the continental realm. To help fill this gap, here we combine the strong dating control and established environmental records provided by stalagmites with molecular temperature proxies derived from microbial membrane lipids. This will allow the study of study climatic changes and their impact on the continental environment within one well dated context.
The proxy used here is the MBT/CBT index, which is based on the degree of branching and cyclicisation in the carbon skeleton of bacterially derived GDGTs, microbial membrane lipids which have been shown to vary in structure with temperature and pH. The relationship between the index and surface temperature has been demonstrated in soils, but not previously studied in stalagmites.
In order to test the robustness of this technique, thirty-eight speleothem samples from twenty-one sites around the world were analysed for MBT/CBT and the results compared with known mean annual temperature. The main outcome is a clear relationship with mean annual temperature, with the correlation showing an r2 value of 0.68, which is slightly lower than, but comparable to, that found in soil work. The data has a large scatter, but analysis indicates that this is partly due to consistent off-set below known temperature at some sites, and may be accountable for in calibrations. The second outcome is that the associated index for terrestrial organic matter input (BIT) is very low, with twenty-three samples showing a BIT below 0.5, of which twelve have a BIT below 0.1. This indicates that the microbial material preserved is principally cave or aquifer derived, with the microbial communities being more analogous to those of aquatic environments than to soils. In light of this, future work will focus on developing better calibrations, twinned with more indepth study of the nature of the microbial source.